Find an equation of joint variation. Then solve for the missing value. r varies directly as s and t. one set of values is r = 28, s = 7, t = 8, Find r when s = 12 and t = 9​

Sarah took the advertising department from her company on a round trip to meet with a potential client. including sarah a total of 11 people took the trip. she was able to purchase coach tickets for $280 and first class tickets for $1280. she used her total budget for airfare for the trip, which was $6080. how many first class tickets did she buy? how many coach tickets did she buy?